I'm trying to understand what each of the 10 data input pins of a DAC mean. The datasheet for the AD9761 I am using is available here.
The pins' description is given as follows (see page 10):
Each DAC consists of a large PMOS current source array capable of providing up to 10 mA of full-scale current, IOUTFS. Each array is divided into 15 equal currents that make up the four most significant bits (MSBs). The next four bits or middle bits consist of 15 equal current sources whose values are 1/16 of an MSB current source. The remaining LSBs are binary weighted fractions of the middle bits’ current sources. All of these current sources are switched to one of two output nodes (i.e., IOUTA or IOUTB) via PMOS differential current switches.
I am confused about a few things here.
"Each array is divided into 15 equal currents" -> Why 15? The four MSBs have 2^4 = 16 combinations.
"equal currents" -> Why are the currents equal? Does it mean that 1000_0000_00 is the same as 0100_0000_00? Why do that?
"The remaining LSBs are binary weighted fractions" -> What is a binary weighted fraction?